Typical inertial systems operate by integrating the outputs of a set of three-axis gyroscopes and accelerometers to obtain attitude and position information of a platform in inertial space. These inertial elements have random drift and other errors which can increase rapidly over time and, as a result, introduce substantial error into the position measurement. To prevent build-up of these errors, a zero update stop (ZUPTS) procedure may be performed periodically to reset velocities, to perform position correction and to correct sensor attitude using the vertical gravity vector and earth rotation speed as references. In general, the accuracy of the measurement is directly related to the frequency of ZUPTS, that is, as the frequency of ZUPTS increases, so does the accuracy of the sensor. To perform a ZUPTS, the inertial platform is brought to rest and sensor velocity readings are zeroed to eliminate velocity error. Also, position error corrections, based on velocity error, can be made. At the same time, attitude adjustment can be performed by comparing the sensor angular measurement with the known gravitational vertical vector. The difference between the two is zeroed to eliminate angular or attitude measurement error. Adjustments can also be made referenced to the speed of rotation of the earth. When the ZUPTS is completed, the sensor can again be moved to begin inertial measurements.
Another form of error correction involves performance of a coordinate update stop (CUPTS) in which position coordinates are updated according to actual known position. In performing a CUPTS, the sensor can be brought to rest at a known position, such as a geodesic marker fixed to the ground. CUPTS can also be performed by receiving position coordinates from a global position satellite (GPS) system.
Inertial systems have been used in surveying systems to provide surveys of large areas of land. Such systems are typically mounted in a motor vehicle or helicopter and are moved over the land being surveyed. To perform a ZUPTS or CUPTS in such systems, the vehicle must be temporarily stopped or the helicopter must be landed on the ground while velocity and angular errors are zeroed. Where large areas are being surveyed, these stops can be inconvenient since they can add considerable time to the procedure. As a result, in such systems, ZUPTS and/or CUPTS are performed infrequently. Since sensor error is dependent on time, this infrequent rate of error correction can introduce substantial errors into the survey. Thus, due to the inconvenience of performing frequent ZUPTS and/or CUPTS, in these prior vehicle-mounted systems, accuracy of position and angular measurement is sacrificed.